The invention relates generally to an electrophotographic printing machine and, more particularly, to a development system which includes a magnetic developer roll for transporting developer material to a development zone; and a magnetic system for generating a magnetic field to reduce developer material bed height in the development zone.
The following application is incorporated herein by reference: patent application Ser. No. 09/004,464, entitled, "APPARATUS AND METHOD FOR NON-INTERACTIVE MAGNETIC BRUSH DEVELOPMENT", which has been filed concurrently.
Generally, an electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to an optical light pattern representing the document being produced. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the document. After the electrostatic latent image is formed on the photoconductive member, the image is developed by bringing a developer material into proximal contact therewith. Typically, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted to the latent image from the carrier granules and form a powder image on the photoconductive member which is subsequently transferred to a copy sheet. Finally, the copy sheet is heated or otherwise processed to permanently affix the powder image thereto in the desired image-wise configuration.
In the prior art, both interactive and non-interactive development has been accomplished with magnetic brushes. In typical interactive embodiments, the magnetic brush is in the form of a rigid cylindrical sleeve which rotates around a fixed assembly of permanent magnets. In this type development system, the cylindrical sleeve is usually made of an electrically conductive, non-ferrous material such as aluminum or stainless steel, with its outer surface textured to improve developer adhesion. The rotation of the sleeve transports magnetically adhered developer through the development zone where there is direct contact between the developer brush and the imaged surface, and toner is stripped from the passing magnetic brush filaments by the electrostatic fields of the image.
Non-interactive development is most useful in color systems when a given color toner must be deposited on an electrostatic image without disturbing previously applied toner deposits of a different color or cross-contaminating the color toner supplies.
It has been observed that the magnetic brush height formed by the developer mass in the magnetic fields on the sleeve surface in this type development system is periodic in thickness and statistically noisy as a result of complex carrier bead agglomeration and filament exchange mechanisms that occur during operation. As a result, substantial clearance must be provided in the development gap to avoid photoreceptor interactions through direct physical contact, so that the use of a closely spaced developer bed critical to high fidelity image development is precluded.
The magnetic pole spacing cannot be reduced to an arbitrarily small size because allowance for the thickness of the sleeve and a reasonable mechanical clearance between the sleeve and the rotating magnetic core sets a minimum working range for the magnetic multipole forces required to both hold and tumble the developer blanket on the sleeve. Since the internal pole geometry defining the spatial wavelength of the tumbling component also governs the magnitude of the holding forces for the developer blanket at any given range, there is only one degree of design freedom available to satisfy the opposing system requirements of short spatial wavelength and strong holding force. Reducing the developer blanket mass by supply starvation has been found to result in a sparse brush structure without substantially reducing the brush filament lengths or improving the uneven length distribution.